This invention relates to a solvent extraction process for separating aromatic hydrocarbons from hydrocarbon mixtures which consist of aromatic hydrocarbons admixed with other hydrocarbons species such as paraffins, branched paraffins, cycloparaffins and/or olefins using cyanoethylated alkoxylated polyol solvents.
It is known that both extraction and distillation techniques have been employed in separating particular hydrocarbon species, e.g., the aromatic hydrocarbons, from petroleum hydrocarbon mixtures having narrow boiling point ranges. For such mixtures, solvent extraction techniques have been employed. These techniques have problems, one of the more significant being the difficulty in choosing a solvent capacity for the aromatic hydrocarbon species to be separated as compared with those hydrocarbon species not desired. Most selective solvents particularly those which are selective for aromatic materials will also dissolve significant proportions of non-aromatic hydrocarbon species.
It is desired to treat the petroleum fractions in such a manner as to separate an aromatic rich stream from the saturated and olefinic aliphatic hydrocarbons. The aromatics have very high octane numbers and are useful for blending into motor gasoline. In addition, such aromatics as benzene, toluene, and the xylenes are valuable feedstocks for a wide variety of uses in chemical industry. The raffinates can be used as components in jet fuel or heating oils or as feed to catalytic reforming. Thus, over the years, there has been a continuing search for solvents which are selective to aromatic hydrocarbons only and have a high solvent capacity for said aromatic hydrocarbons and, at the same time, dissolve very little, if any, of the non-aromatic hydrocarbon species.
A number of selective solvents have been proposed and described for the extraction of aromatic hydrocarbons from mixtures of aromatic and non-aromatic paraffins, olefinic and naphthenic hydrocarbons. For example, U.S. Pat. Nos. 2,568,159 and 2,568,176 disclose the use of cyanoethylated ethylene glycol and diethylene glycol for this purpose. The use of nitrile solvents is also mentioned in U.S. Pat. Nos. 2,458,067; 2,842,484; 3,372,109 and 3,436,437. The use of 1,2,3-tris(2-cyanoethoxy)propane as a selective solvent is disclosed in U.S. Pat. No. 3,860,512.
The solvents of this invention are more heat stable than the cyanoethylated diols and triols of the prior art.
The use of sulfolane is disclosed as a selective extractant to improve the selectivity of separation in U.S. Pat. No. 2,407,820. There are, however, several drawbacks associated with the use of sulfolane as a selective solvent in hydrocarbon extraction processes. For example, in conventional solvent extraction processes, an extract phase containing the more readily soluble component is recovered by treating the starting mixture with the selective solvent and using a liquid-liquid extraction process. The solvent is thereafter recovered from the extract as the bottoms in a distillation operation. Sulfolane, however, readily degrades at its atmospheric boiling point. Therefore, it has been found necessary to use sub-atmospheric pressure in the separation of sulfolane from the remainder of the extract phase. Furthermore, sulfolane has a relatively high freezing point 82.degree. F, this necessitating steam heating of the lines and equipment carrying the pure solvent in order to prevent its solidification. The necessity of using elevated temperatures when handling the pure solvent and the sub-atmospheric pressure required for the separation of the dissolved components from the sulfolane results in increased capital equipment costs and higher energy requirements.